There have been well known techniques where the driving posture of a driver, in particular, the behavior of the driver's head is stabilized to suppress a disorder of steering operation by the driver when a vehicle turns. For example, there is a method disclosed in patent document-1, in which a sight adjuster is arranged with a configuration that uniformly slopes down from about a central position of a vehicle toward both vehicle-transverse ends, to mask a lower end portion of a front windshield of the vehicle, thereby suppressing the tilt of a fore sight of a driver when the vehicle turns, in order to stabilize the driver's driving posture for prevention of a disorder of steering operation.
The steering operation by a driver when turning a vehicle, it is known that a difference exists between a clockwise steering speed in a right turn and a counterclockwise steering speed in a left turn. For the right-hand drive vehicle for example, the clockwise steering speed is generally greater than the counterclockwise steering speed, with a remarkable tendency if the driver has little experienced. Such being the case, a difference in a steering speed between right and left turn exists, which may cause a disorder of steering operation that the known method such as disclosed in the patent document-1 is unable to prevent.    [Patent document-1] Japanese Patent Application Laying-Open Publication No. 2005-75187